This invention relates in general to ultrasonic measurement systems. More specifically, it relates to an ultrasonic system that (1) utilizes obliquely incident, vertically polarized shear mode (SV) energy propagated in a solid to measure the acoustic impedance and related properties of an adjacent fluid, typically a liquid, and (2) in its preferred form measures a selected parameter of the fluid--its impedance or level--while rendering the measurement substantially insensitive to variations in other parameters affecting the transmission of the interrogating sound wave.
In both scientific and industrial applications ultrasonic techniques have been used to measure the acoustic impedance Z of a liquid or related parameters such as liquid level in a container, density in a single and two-phase liquids, and mass flow rate. In a single phase liquid, the density is given by Z/c, the impedance divided by the sound speed. The mass flow rate of liquids such as aircraft fuel is proportional to the product of the impedance and the fuel Mach number V/c, the flow velocity divided by the sound speed. The Mach number is readily determined by standard ultrasonic techniques using Doppler flowmeters, beam drift flowmeters or upstream/downstream contrapropagating transmission flowmeters.
Many common commercial systems for measuring impedance or related parameters utilize longitudinal (L) ultrasonic energy directed at a normal incidence to the fluid under interrogation. In one system a single beam of L wave energy is directed vertically downward into a container partly filled with a liquid to measure the liquid level, but confusion arises if foam is above the liquid. Another intrusive system, used principally to detect liquid level, transmits L wave energy between transducers across a rectangular recess formed in a side of a probe when the recess is immersed in the liquid. Energy is not transmitted when air or foam fill the recess. This device thus provides comparatively crude "liquid present" or "liquid not present" information. This prior art system is of course limited to applications where the intrusive transducers and their associated seals, bonds and electrical connections can operate in the environment in and around the liquid which can include extremely high degrees of temperature, pressure and radiation or chemically corrosive substances.
In two 1955 articles J. Kritz describes a mass flowmeter that utilizes the reflection coefficient of L waves at normal incidence to an interface of a quartz transducer and aircraft fuel to measure the impedance of the fuel. The fuel "loads" the transducer either directly by immersion or indirectly through a plate of low attenuation material as described in his U.S. Pat. No. 2,869,375. A similar approach is described by W. Welkowitz in U.S. Pat. No. 2,959,054. One of the present applicants also developed techniques for measuring acoustic impedance with longitudinal waves propagated in special buffer rods or thin walls to interrogate a liquid at a liquid-solid interface with normal incidence. One problem with these techniques is that the transducer and/or its backing interacts with the reflected wave too often, extracting a significant fraction of the energy. As a result, the measurement is too sensitive to variables associated with the transducer and its coupling. Examples of configurations of this type are described in "Industrial Applications of Ultrasound-A Review II. Measurements, Tests and Process Control Using Low Intensity Ultrasound" by L. Lynnworth in IEEE Transactions on Sonics and Ultrasonics, Vol. SU-22(2) pp. 71-101, March, 1975. Pages 82 and 83 of that article also describe the work of Moore and McSkimin using normal incidence shear wave complex reflectance. Additional general references on ultrasonic liquid level approaches include J. and H. Krautkramer, Ultrasonic Testing of Materials, 2nd Ed., Springer-Verlag, pp. 569-572 (1977) and Mason and Thurston (ed.), Physical Acoustics, Vol. 14, pp. 458-462 (1979), contained in Chap. 5 which was authored by one of the applicants.
U.S. Pat. No. 2,996,058 to McSkimin describes the use or horizontally polarized shear mode (SH) waves propagated in a trapezoidally-sectioned block of fused silica to measure the viscosity and elasticity of a liquid specimen resting on a top surface of the block. The SH waves are incident on the top surface at an incidence angle of approximately 79 degrees. The phase shift and attenuation of these waves, due to absorption in the liquid, provides the measure of its viscosity. This system has numerous drawbacks. The top surface of the block must be very flat and smooth and the liquid sample volumes are small and the interaction between the ultrasonic energy and the liquid is confined to a single, small area. These limitations make the apparatus practical only for laboratory use. Further, the McSkimin arrangement is not useful for the many liquids with low viscosities, that is, less than 1 centipoise, unless very high frequencies are used, e.g. approximately 10 to 100 MHz. At these frequencies, however, particularly the more useful ones closer to the upper end of the range, attenuation due to probe losses becomes significant. While McSkimin uses a second interrogating beam which can be SV mode energy, this beam is merely a reference beam to compensate for troublesome experimental (temperature) variations; it does not necessarily interrogate the liquid sample.
Shear waves propagated in magnetostrictive probes having sensors in the form of a helix, circular tube or solid rectangular rod have been used by A. E. Arave to investigate the attenuating affects of surrounding water at various temperatures. This work is reported in "Idaho Nuclear Corporation Reports, IN-1441 and 1442 (1970)" and "Aerojet Nuclear Corp. Report ANCR-1047 (1972)." These probes, however, are characterized by cross sections that are small (1 or several mm) compared to the wave-length and the sound energy, if in the shear mode, is at substantially normal incidence. Because of these small probe dimensions, the interrogating energy is a guided wave, that is, the velocity and mode of propagation of the wave depends on the boundary conditions, in contrast to bulk waves which are not so affected. While Arave's arrangements respond to the acoustic impedance of the liquid, the interaction is unpredictable because of uncontrolled mode conversion (due to sharp curvature) and other uncontrolled losses due to the immersion of a guided wave having an ill defined angular arrangement or impure modal content. The results were also found to be undesirably sensitive to transduction efficiency, which is temperature-dependent.
In contrast to the foregoing systems characterized by interrogation at normal incidence or ultrasonic energy in the longitudinal or SH modes, U.S. Pat. No. 3,512,400 to Lynnworth describes the use of SV mode waves that follow a zigzag pattern in a solid. This system, however, is designed to detect flaws in the solid, not to interrogate a fluid adjacent the solid. Moreover, in flaw detection is is often desirable to operate even beyond the second critical angle and with short bursts of energy, typically a few cycles in duration.
Canadian Pat. No. 648,023 to Van Valkenburg and the Krautkramer book, cited above at p. 572, describe another ultrasonic measuring system using sound waves (which could be in the SV mode) to measure liquid level in a closed container. The illustrated system has a probe that is suspended vertically in the container and transmits the interrogating wave along a zigzag path. This arrangement, however, provides an accurate, unambiguous measure of liquid level only if the operating conditions are relatively ideal, that is, if the liquid and the probe are at a nearly constant temperature, the identity and characteristics (e.g. impedance, viscosity) of the liquid do not change, and other factors such as the residue deposits on the probe remain constant. Under non-ideal conditions, which are typical in commercial applications, the Van Valkenburg system will not produce reliable readings. For example, a 25% change in the impedance of the liquid held in the container can result in a 25% error in the measured level; seasonal, altitude or geographically induced changes in temperature T can result in comparable errors in the measured level (e.g., aircraft fuel tank liquid level). The Kritz article, for example, presents a graph showing large changes in fuel sound speeds as T is varied. Further, in a paper by Korycki et al., Ultrasonics 17, 166-174, July 1979, the acoustic impedance of a petroleum product, Hydrol 40 oil, is plotted in FIG. 12 as a function of pressure and temperature, for pressures from 0 to 200 MPa, and temperatures from 0 to 80.degree. C. This graph shows that Hydrol's impedance varies over a 2:1 range, from 0.105 to 0.215 MN s m.sup.-3. An uncompensated or not desensitized liquid level probe, calibrated for midrange impedance, could easily yield liquid level readings with 40 to 50% errors at impedance extremes.
The Van Valkenburg patent, moreover, does not appreciate that a zigzag propagating SV wave will couple with the fluid to provide a reliable measure of the impedance of the fluid. In the Van Valkenburg arrangement, the measurement does not distinguish between attenuation due to the impedance of the probe, the impedance of the adjacent liquid(s), impedance ratios, or other factors such as variation in the transducer coupling or residues on the probe. This is reflected in the failure of this patent to provide general and sufficient guidelines for designs favorable to this coupling such as the frequency or other (e.g., polarization) characteristics of the wave, limits on the angles of incidence of the wave at the solid-fluid interface, and the structure of the probe, particularly for practical use in non-ideal conditions.
It is therefore a principal object of this invention to provide a practical ultrasonic system for measuring fluid impedance or liquid level that has an unambiguous, predictable and sensitive response.
Another principle object of the invention is to provide such a measuring system that isolates the measurement of a selected parameter and insulates the measurement from or compensates for variations in other parameters.
Another object is to provide a system for measuring two or more variables simultaneously or sequentially, such as fluid impedance, level, temperature, viscosity or viscosity range.
Another object is to provide an acoustic measuring system that does not immerse the transducers and which can interrogate an inaccessible fluid, fluids in one or two phases, and fluids having a wide range of viscosities.
Still another object is to provide a measuring system that can be readily designed for either a localized or distributed interrogation of a fluid at one or several surfaces and interrogation along paths with varying orientations.
A further object is to provide a measurement system that is readily applied to existing conduits and containers, including conduits and containers with curved walls.
Another object is to provide a measuring system that simplifies the compensation for changes in the probe such as its temperature.
Yet another object is to provide a measuring system that does not require special surface preparation and operates effectively despite thin layers of residue, corrosion, paint or the like adjacent the fluid under interrogation.
A still further object is to provide a measuring system that can measure liquid level under static or dynamic conditions including rapid fluctuation or turbulence.
Another object is to provide a measuring system that operates at frequencies that are not readily attenuated in common liquids.